/Engine/Plugins/Media/MediaCompositing/Source/MediaCompositing/Public/MovieSceneMediaSection.h
If there is a proxy, get the media source from it. Otherwise return our media source.
[UMediaSource](API\Runtime\MediaAssets\UMediaSource) * GetMediaSourceOrProxy ( [IMovieScenePlayer](API\Runtime\MovieScene\IMovieScenePlayer) & Player, [FMovieSceneSequenceID](API\Runtime\MovieScene\FMovieSceneSequenceID) SequenceID ) const